The quality or efficacy of perishable packaged foods, photographic film, biologicals, chemicals, pharmaceuticals and other consumable materials is usually dependent on their time and temperature of storage. In an effort to assure the consumer of the freshness of perishable goods, the supplier often stamps expiration dates on these goods to indicate the end of the useful life of the goods. Although the expiration dates usually are conservative estimates of shelf life at recommended storage temperatures, they do not take into account the substantial reductions in shelf life of a product caused by storage at temperatures higher than those recommended that, for example, might be experienced transiently during power outages and breakdowns in refrigeration. High temperatures might also be experienced by individual packages in warm sections of refrigerated showcases or storage compartments or by packages in refrigeration units equipped with inaccurate thermostats. Finally, unaccountable removal and replacement of food, pharmaceuticals and other labile materials from the refrigerator for purposes of transport and use make it difficult to predict the remaining useful lifetime of individual packages of perishables. These uncertainties often lead to premature disposal of perishables or the use of deteriorated perishables. Such practices which can be wasteful of material and labor and in some cases cause a health hazard, could be minimized by devices that would indicate whether perishable goods had accumulated a deleterious amount of thermal exposure.
Many devices have been described for indicating the expiration of shelf-life at a predetermined temperature. Representative devices are those described, for example, in U.S. Pat. Nos. 3,966,414, 3,999,946, 4,057,029, 4,212,153, 4,278,561, 4,298,348 and 4,384,980. Generally, these devices rely on chemical reactions or diffusion of indicator dyes through a semipermeable membrane. A serious limitation of most known time-temperature indicators based upon nonenzymatic chemical reactions is their sensitivity to light or impurities such as oxygen, trace metal ions, and other free radical scavengers which contaminate most solvents and chemical reagents. Devices based upon enzymatically catalyzed reactions are likely to be limited by deterioration of the enzymes at rates which are difficult to control. These limitations can affect the reliability of time-temperature indicators based on polymerization of acetylenes, those based on the bleaching of dyes by peroxides and those based on enzymatically catalyzed reactions. The use of diffusion through a permeable plastic membrane also is limited since this process like most diffusion based processes shows little dependence upon temperature. The limitation is partially overcome in devices comprising a diffusion layer capable of transition between an impermeable and permeable liquid crystal state. Above the transition temperature, the devices should indicate the passage of time in a fairly temperature independent fashion characteristic of a diffusion process. Below the transition temperature, little or no diffusion occurs and little or no change in the indicator occurs with passage of time. Although these devices appear to be good indicators for signalling transient exposure to temperatures above the melting point of the membrane, they do not appear to be well suited for reflecting thermal aging of goods which may be exposed to temperatures between 3.degree. and 50.degree. C. during their lifetimes.
It therefore is an object of the present invention to provide improved means for indicating accumulated thermal exposure (ATE) as well as methods of use for indicating the expiration of the useful life of perishable goods and the like.
It also is an object of the invention to provide exposure indicator means yielding a relatively abrupt visible change after a predetermined accumulated thermal exposure has been obtained or yielding a continuously increasing color intensity that is relatable to the accumulated thermal exposure.
Another object of the invention is to provide a time-temperature indicating device that, prior to use, is in a standby mode and, for use as a monitoring device, can be activated to coincide with the onset of aging of the goods to which it is affixed.
Still another object is to provide a device that is responsive to accumulated thermal exposure and affords controlled release of an agent such as a flavor or fragrance.
These and other objects, features, and advantages will be seen from the following description of the invention and by reference to the accompanying drawings and examples.